Electrochemical Performance and Capacity Fading Mechanism of LiFePO₄ at Different pH Aqueous Electrolyte Solutions

The electrochemical stability of LiFePO₄ in a Li⁺-containing aqueous electrolyte solution is critically dependent on the pH value of the aqueous solution. It shows a considerable decay in capacity of LiFePO₄ upon cycling when the pH value is increased to 11. The mechanism responsible for the capacity fading is extensively investigated by means of cyclic voltammogram, ac impedance, charge/discharge, ex situ X-ray diffraction, and chemical analysis. LiFePO₄ is relatively electrochemically stable in LiNO₃ aqueous solution with pH=7. But the electrochemical performance of LiFePO₄ in aqueous electrolyte is inferior to that in organic electrolyte. It is attributed to the loss of Li and the Fe, P dissolution during prolonged charge-discharge in aqueous medium. A precipitate is formed on the surface of LiFePO₄ electrodes. It results in the change of crystalline structure, a large electrode polarization, and capacity fading.